Digital filtering and detection techniques have enhanced signal processing with the aid of faster hardware and efficient softwave to provide a logic signal indicative of a desired energy within the passband and to preserve the signal amplitude. A particular digital detector is also disclosed by R. A. Lane and D. K. Fronek in U.S. Pat. No. 4,047,114 issued Sept. 6, 1977. The detector is responsive to a broad spectrum of input frequencies to detect the presence of a selectable frequency with specified bandwidth. The bandwidth logic and center frequency logic of the decoder may be adjusted across a broad frequency range. Incoming continuous signals are received by a wave shaping circuit (Schmitt trigger), to change a slow moving waveform such as a sine wave or triangular wave to a square wave whose period is processed digitally by a gate circuit and multivibrator circuit.
As the input crosses a threshold, the wave shaping circuit produces an ON level of signal that represents the period of the incoming signal. A gated oscillator (pulse generator), which is running many times faster than the input signal, provides the counting signal that is processed by an up-counter. The gate circuit provides a stable window of gate pulses representing the period of the input waveform. The gated pulses are counted by the up-counter. This counter may be any length or any radix, such as binary or binary coded decimal. The output of the counter is fed to a decoder for final processing.
The multivibrator circuit performs administrative timing control. When the output of the wave shaping circuit falls, a monostable is triggered, this monostable provides a timed pulse to a latch to store the count. At the end of the pulse produced by the first monostable, it's falling edge causes a second monostable to generate a pulse to clear the up-counter for new data. The latch temporarily stores the counter data for processing by the combinational decoder module. The output from this module indicates the desired center frequency energy in the filter. Additional outputs, f.sub.L and f.sub.H, are optionally provided to permit a bandwidth tolerance.